PROJECT SUMMARY Organisms exploit predictable environmental light/dark cycles by systematically varying their metabolism, physiology, and behavior in synchrony with day and night. These circadian rhythms, which are produced by molecular clocks, can have profound consequences to health and disease if disrupted. However, the mechanisms of these circadian clocks are only partially understood in any organism. Be- cause a rigorous understanding of these mechanisms will be indispensable for tackling circadian- related diseases, the long-term goal of the LiWang research group is to elucidate the mechanisms of clocks and clock control over cellular processes in model systems. Reports in the literature and data from this laboratory strongly suggest that clock proteins rearrange their global conformations, and thus their functional properties, as part of the timekeeping mechanism. For example, pace-setter proteins, PER in animals and FRQ in fungi, undergo conformational changes between globally compact and open states as they keep time. Similarly, we recently discovered that the cyanobacterial clock protein, KaiB, also executes global conformational rearrangements, KaiB ? KaiB*, called ?fold switching?. Surprising- ly, we also found that KaiB fold switching not only plays an essential role in the generation of circadian rhythms, but regulates the transmission of those rhythms downstream. Thus, the objective here is to elucidate the roles of large-scale conformational changes by proteins in clock mechanisms. Attaining this goal is predicted to have an enormous influence on the field of both prokaryotic and eukaryotic chronobiology. The central hypothesis of this proposal is that KaiB ? KaiB* fold switching is the linch- pin that joins oscillator function to clock output. To test the central hypothesis we will pursue three spe- cific aims: 1) Establish the role of KaiB ? KaiB* fold switching in oscillator function; 2) Determine how KaiB ? KaiB* fold switching regulates the SasA output pathway; and 3) Determine how KaiB ? KaiB* fold switching regulates the CikA output pathway. The central hypothesis is strongly supported by pre- liminary data obtained by using an integrative approach: structures ? mutants ? in vitro experiments ? computational modeling ? in vivo experiments. A strong team of collaborators with expertise in each area enhances the feasibility of the work proposed. This proposal is innovative, because a lack of reports in the literature reveals that the central concept of this proposal has been overlooked: Large changes in protein structures underpin the mechanisms of circadian clocks. The proposal is significant, because the findings here are expected to open new and actionable insights into eukaryotic clocks, and to other processes in which protein fold switching may not have been previously recognized. Ultimately, such knowledge has the potential to create strategies with which to tackle circadian-related diseases.